Use of an organic electro-luminescence element (hereinafter, referred to as “organic EL element”) as a light source of a lighting device is proposed.
When an organic EL element is short-circuited, a voltage Vf corresponding to an impedance between the anode and cathode of an organic EL element is applied between anode and cathode. The voltage Vf is lower than the voltage between the anode and cathode of an organic EL element in a normal state.
Failure detection means described in Patent Literature 1 measures an electric potential Vf of an anode electrode of an organic EL element. When the electric potential Vf of the anode electrode is lower than a reference voltage, the failure detection means of Patent Literature 1 detects a short circuit failure of the organic EL element.
When the organic EL element is short-circuited, an electric current flowing into the organic EL element increases since the impedance between the anode and cathode of the organic EL element decreases.
Failure detection means described in Patent Literature 2 measures the voltage of a resistance connected to an organic EL element in series. Increase in current can be detected by the measured voltage. The failure detection means described in Patent Literature 2 therefore detects a short circuit failure when the measured voltage exceeds a predetermined threshold.
A short circuit failure of the light emitting element can be detected by the measurement of the voltage Vf (output voltage) between the anode and cathode of the light emitting element.
A circuit protector (failure detection means) described in Patent Literature 3 comprises a first comparator and a second comparator. The first comparator outputs a high level voltage when the output voltage of an LED (LIGHT EMITTING DIODE) is larger than a first reference voltage. The first comparator outputs a low level voltage when the output voltage of an LED is smaller than the first reference voltage. The second comparator compares the output voltage of the first comparator with a second reference voltage. The second comparator outputs a low level or a high level voltage depending on the comparison result. The circuit protector detects a short circuit failure from the output voltages of the first comparator and the second comparator.
The above-mentioned failure detection means measures the voltage Vf between the anode and cathode of a light emitting element, or a voltage associated with the Vf. A short circuit failure of a light emitting element is then detected from the measured voltage and the reference voltage.